CA2194753A1

CA2194753A1 - Electrochemical peroxide generator and process for making peroxide

Info

Publication number: CA2194753A1
Application number: CA002194753A
Authority: CA
Inventors: Mark E. Fraser; Alan S. Woodman; Everett B. Anderson; Jennings E. Taylor
Original assignee: Individual
Current assignee: Physical Sciences Corp
Priority date: 1994-07-15
Filing date: 1995-07-14
Publication date: 1996-02-01
Also published as: US5565073A; EP0799328A4; US5647968A; EP0799328A1; US5695622A; WO1996002686A1

Abstract

In the disclosed electrochemical cell (10) for the production of an alkaline solution of peroxide, especially on site production, the electrolyte is divided into an aqueous alkaline catholyte (19) and an aqueous alkaline anolyte (17), and the cathode (37) is a gas-diffusion electrode. The active material of the electrolyte side (33) of the gas-diffusion cathode comprises a particulate catalyst support material having a surface area of about 50 to about 2,0002m/g, and deposited on the particles of this support material, 0.1 to 50 weight-%, based on the weight of the active layer, of gold or gold alloy particles having an average size 40 but less than about 200.ANG.. These gold or gold alloy particles are substantially selectively catalytic for the reduction of oxygen to peroxide (e.g. HOO-). The electrolyte flow patterns are designed to avoid loss of peroxide resulting from oxidation at the anode (39).
In the operation of the cell, a product with a hydroxyl:perhydroxyl ratio less than 2:1 can be obtained.

Description

~ WO 961026R6 P~ J,'~,. 1.~.3 4 '1 ~J 3 1 EE-cl~ocHEMIcAL PEROXIDE GE~ERATOR AND PROCESS FOR MAKING P~ROXIDE
' 2 3 Fleld of th~ Invention 4 This invention relates to electrochemical cells designed to synthesize peroxide IH00-, ~2-~ or H202 dissolved in an 6 alkaline medium) by cathodic reduction of an oxygen-~ont~inin5 7 gas and to processes for operating such cells. An aspect of 8 this invention relates to electrochemical cells for the g syr.thesis of peroxide wherein the electrolyte is divided into an aqueous ~lk~lin~ catholyte and an aqueous ~1 k~l ine anolyte.
:1 Another aspect of this invention relates to a process for 12 syr.ehesizing peroxide which can be operated at relatively low 13 cell voltages and relatively high current densities and 14 efficiencies.
Description of th~ Prior Art 16 It has long been known that hydrogen peroxide can be 17 syr.thesized electrochemically, taking advantage of modern 18 advances in electrochemical cell technology. The patent 19 lieerature published on this subject in the late 1960's and early 1970's took into consideration the possibility of using 21 a gas-diffusion cathode. A "gas-diffusion electrode~ is 22 normally considered to comprise a structure which is gas 23 permeable on one major surface ~sometimes called the "gas 24 side") and electrocatalytic on the opposite major surface, which opposite surface is in contact with the electrolyte and 26 is sometimes called the "electrolyte side~. The electrolyte 27 is permitted to pel, ~e into the electrolyte side to a degree 28 sufficient to provide a multi-phase interface between a 29 gaseous reactant, a solid electrocatalytic material, and the electrolyte Iwhich is generally a liquid). However, 31 significant permeation of electrolyte through pores or 32 interstices within the catalytic material of the gas-diffusion 33 electrode at a significant flow rate is neither necessary nor ~UBS~lTUTE SHEET (RULE 261 wos~/02686 r~v~ . ~s~
~ ~ ql~ 7~;3 1 desirable.

2 For a representative sampling of disclosures from this 3 late-1960's/early 1970's period, see the several U.S. patents 4 issued to Grangaard, e.g. 3,459,652 (August 5, 1969), 3,462,351 (August 19, 1969), 3,507,769 (April 21, 1970), 6 3,592,749 (July 13, 1971), and 3,607,687 (September 21, 1971).
7 These disclosures typically contemplate a generally free-8 flowing catholyte which takes up peroxide (generally in the 9 form of HOt- dissolved in the alkaline cathclyte) and is withdrawn from the cell for the purpose of recovering a 11 product which is intended to be directly useful in industry, 12 e.g. as an alkaline bleach solution.
13 Experts in the electrochemical synthesis art found the 14 performance of the Grangaard cells to be very disappointing, however, and by the mid-1970's, even the fundamental 16 principles upon which the Grangaard concepts were based were 17 being called into question. For example, according to Oloman 18 and his coworkers, see U.S. Patents 3,969,201 and 4,118,305, 19 issued July 13, 1976 and October 3, 1978, respectively, the Grangaard cells produced an aqueous alkaline product having a 21 peroxide concentration of only about 0.5~ with an NaOH/H2O2 22 ratio (by weight percent) vf 4:1 (cf. ~.S. 3,459,652). As is 23 known in the art, some use.s of bleaching solution, e.g. in the 24 pulp and paper industry, generally call for much higher concentrations of peroxide and~or for NaOH/H2O2 ratios in the 26 range of abvut 1:1 to about 2:1. O].oman et al, among others, 27 questioned the basic idea of utili~ing a gas-diffusion cathode 28 of the classical structure wherein catholyte merely permeates 29 into the electrode structure from the electrolyte side. Thus, by the mid- to late lg70's, prior art workers were directing 31 their attention to cathode structures constructed from a 32 fluid-permeable, electrically conductive mass (e.g. a bed of SUBSTITUTE SHEET ~ULE 26~

~ W096/02686 f~ ~4, r)3 1 conductive catalytic particles or a fixed, porous conductive 2 catalytic matrix) with sufficient porosity to permit a 3 constant trickle or flow of electrolyte through the entire 4 volume (or most of the volume) of the cathode mass In an electrochemical cell provided with such a fluid-permeable, 6 electrically conductive cathode mass, the cathode can, if 7 desired, fill up the entire cathode compartment, so that all 8 or most of the catholyte is confined to the interior of the 9 cathode mass.
In subsequent developments based upon the packed-bed or 11 porous matrix concept of a cathode, the cathode was in some 12 cases placed in contact with a non-conducting porous matrix 13 (such as a felt) or was employed in a cell having in essence 14 a single electrolyte rather than an electrolyte divided into catholyte and anolyte.
16 In many patent disclosures illustrating the packed-bed or 17 porous matrix concept of a cathode, the product (generally an 18 ~lk~l ;n~ solution of peroxide, most typically catholyte which 19 has been passed through the cathode mass) is collected from an end or edge or other portion of the cathode mass, rather than 21 from a generally free-flowing catholyte which has merely 22 contacted and/or permeated to some degree a surface of the 23 cathode. Alternatively, the product is essentially catholyte 24 which has wicked through a non-conductive, porous mass such as a felt which is in contact with the cathode.
26 Peroxide-generating cells containing packed-bed or 27 porous-matrix cathodes have in recent years become 28 commercially available for use as on-site peroxide g~n~r~t~rS, 29 and considerable effort has gone into the optimization of their performance. However, these commercially available 31 cells operate at overall cell potentials (Ec,!~) of about 2.0 32 ~ and current densities not significantly ~Y~P~1ng 60 amperes SU~SrITlJTE SHEET (RULE 26) W096/02686 PCT~Isss/09733 2 ~ ; 3 1 per square foot (about 64.5 mA/cm = 645 A/m2~. Even assuming 2 a current efficiency of 85 to 30%, a large amount of electrode 3 surface area is required in a typical commercial installation, 4 resulting in higher capital costs to the industrial user.
Moreover, the packed-bed or porous matrix concept of 6 cathode construction has not provided any i~ vel.lent in 7 quality control as compared to cells utilizing gas-diffusion 8 cathodes. The packed bed or porous matrix can develop "hot g spcts" in which current densities, etc. are higher than ~0 average for the bed or matrix as a whole, thereby creating the ~1 risk that some part of the cathode might become "starved" for 2 three-phase interface sites and can place the entire bed or ~3 matrix at risk of catastrophic failure. This risk can be ~4 reduced through the use of a significant stoichiometric excess of oxygen, but non-uniform consumption of oxygen throughout 6 the cathode bed or matrix 9till contributes to poor quality 7 control. In addition, bipolar ce].l construction, with 8 stacking of cells for more efficient overall operation, is i9 problematic, due to the variability in the performance of ~0 individs~al cells.
~1 Accordingly, despite significant advances in the field of 72 on-site electrosynthesis of peroxide over the last twenty ~3 years, cell performance is still in need of substantial ~4 il;l~JLi_/VI

SU~STITUTE SHEET lRULE 26~

~ W0 ')61026R6 r~ uo~
~ 9~7~i3 2 It has now been discovered that the performance of 3 electrochemical peroxide synthesis cells and processes in 4 terms of operating potentials, range of current densities, S cathode lifetime, and the resulting system capital costs can 6 be markedly improved through the use of an electrochemical 7 catalytic material ~ont~;n;~i~ gold particles and through the 8 use of a generally free-flowing catholyte which inhibits loss 9 of peroxide due to oxidation of perhydroxyl ion at the anode of the cell. Owing to a unique cathode preparation process 11 incorporating the gold catalyst, the relatively poor 12 performance of the Grangaard cells is not observed during the 13 practice of this invention.
14 Thus, an electrochemical cell of this invention comprises:
16 a partitioning means (such as a fluid-permeable 17 separator) for partitioning the cell into an anode compartment 18 and a cathode compartment, the anode compartment cnn~;ning an 19 anode and an aqueous alkaline anolyte, a cathode compartment defininy a space containing a 21 generally free-flowing aqueous ~lk~l;ne catholyte, a gas-22 diffusion cathode having two major surfaces ~the cathode 23 occupies, at most, only a minor proportion of the volume of 24 this space), the space defined by the cathode being constructed and arranged to permit generally unrestricted flow 26 of the aqueous alkaline catholyte across one of the major 27 surfaces of the gas-diffusion cathode, i.e. the "electrolyte 28 side" (the other major surface of the cathode, i.e. the ~gas 29 side~, is in contact with oxygen-containing gas). A catholyte withdrawal means permits withdrawal of the spent catholyte, 31 which is the desired product of peroxide dissolved in an 32 aqueous alkaline medium.

SUBSTITUTE SI~EET (R~LE 26 w096/02686 2}~47~3 rc~

1 The electrolyte side of the cathode comprises an 2 electrochemically active material which is composed of:
l a particula~e catalyst support material having a 4 surface area, by the B.E.T. method, of about 50 to about 20Q0 m2/g, 6 deposited on the particles of catalyst support 7 material of the active layer, 0.1 to 50 weight-%, based 8 on the weight of the active layer, of a particulate 9 elemental metal comprising gold particles having an average size, measured by transmission electron 11 microscopy, which is greater than about 4 but less than 12 about 20 nanometers, this particulate elemental metal 13 being substantially selectively catalytic for the 14 re~ncti~n of oxygen to peroxide ion or hydrogen peroxide, and 16 preferably, a gas-permeable, electrically conductive 17 support material on which the active material is 18 deposited. The active material is preferably rendered 19 hydrophobic by including in it at least 30~ by weight, based on the weight of the active layer, of hydrophobic 21 polymer.
22 Electrical leads in electrical contact with the anode and 23 the cathode of the ce].l are provided for the external 24 electrical circuit. If desired, the cell can be bipolar and can be electrically c~nn~rr~ to one or more additional cells.
26 In the process of operating this electrochemical cell, 27 the overall cell potential ~Ec~ll) need not exceed 2 volts and 28 can be less than 1.5 V; current densities can range from 700 29 to 2,000 A/m2 (70 to 200 mA/cm2) or more, yet current efficiencies do not suffer and are typically in the range of 31 about 85 to about 95~. In addition, current densities in 32 excess of 300 mA/cm2 are attainable. Since the size, and SIJBSTITUTE SHEET (RULE 26) ~ ~/0 gG10268~ r~ u................................... ,!~
;~1 94~ ~3 1 therefore the cost, o~ a commercial system is a direct linear 2 function of the electrode area, a substantial ; , ~.ment in 3 sustainable current density will siynificantly reduce the 4 system capital costs. This outstanding performance can be obtained under near ambient operating conditions, including 6 temperatures in the range 35-40~C. The hydroxide/peroxide 7 ratio of the resulting product is well-controlled but variable 8 in accordance with the desired use and hence can vary from 9 about 1.6:1 to all higher values. The peroxide yields associated with these product ratios are typically 3-5 wt%.
11 BRIEF DESCRIPTION OF THE 5RAWIN&
12 In the ac~ . nying Drawing, wherein like reference 13 numerals denote like parts in the various I b~ qt5 of the 14 invention, FIGURE 1 is a schematic view of a relatively simple 16 ~mho~; of an electrosynthesis cell of this invention which 17 employs an anolyte-to-catholyte flow and therefore requires 18 only one electrolyte inlet and one electrolyte outlet, 19 FIGURE 2 is a schematic view, similar to Figure 1, of an especially high-performance ~ ; t of an electrosynthesis 21 cell of this invention which employs anolyte and catholyte 22 flows which are generally separate but are in fluid 23 communication with each other, and 24 FIGURE 3 i9 a greatly enlarged fragmental cross-sectional view of the cathode 30 shown in Figure 1.

27 Turning first to the Drawing, FIGURE 1 illustrates an 28 emho~ r~ of an eleccrochemical synthesis cell 10 of this 29 invention which is desirable from the standpoint of simplicity. Cell 10 comprises an anode 11, a gas-diffusion 31 cathode 30, a fluid-permeable cell divider or separator 15, an 32 anode lead 39, a cathode lead 37, an inlet 21 for the flowing SlJBSTITUrE SI~EET ~RULE ~6) W096/~26~6 ~ l q~7 ,. 5 3 PCT~895/~9733 l electrolyte, an outlet 23 located near anode 11 for the 2 release of oxygen generated during cell operation, an outlet 3 25 for the flowing electrolyte, an inlet 27 located near the 4 ~gas side" of cathode 30 for the introduction of an oxygen-c~rt~in;ng gas, and an outlet 29 for excess oxygen-~Ont~in;~S
6 gas. The electrolyte introduced in to inlet 21 is supplied 7 from a source 12 external to cell lO.
8 Separator lS divides cell 10 into an anode compartment 9 17, which is constantly filled with continuously flowing anolyte, and cathode compartment 19, which is constantly 11 filled with continuously flowing catholyte, but separator 15 12 is porous and has sufficient porosity to permit flow-through 13 of electrolyte and hence migration of ions between 14 compartments 17 and 19. Cathode 30 ~see also ~IGURE 3) has a typical gas-diffusion electrode structure comprising an 16 electrically conductive sheet-like gas-permeable support 17 material 31 upon which is superposed an electrocatalytically 18 active material 33 comprising high surface area particles upon l9 which tiny particles of gold or a gold alloy have been deposited. Active material 33 preferably also contains a 21 hydrophobic binder material, e.g. a highly fluorinated 22 olefinic polymer or other polyhalohydrocarbon such as 23 polytetrafluoroethylene~ in an amount greater than 25% by 24 weight, based on the weight of the active material 33, most preferably about 50 to 70 weight-S. Amounts greater than 26 about 75 or 80 weight-S can have an unacceptable adverse 27 impact upon per~ormance without increasing cathode li~e 28 significantly as compared to the 70 weight-S level of binder.
29 Support material 31 can be a carbon cloth, carbon paper, or teflonated metal screen which serves as a current collector 31 and which is su~iciently hydrophobic (or has been treated 32 with a hydrophobic polymer such as a polyhalohydrocarbon e.g.

SUE~STITUTE SHEET (RULE 2~1 ~ ~'096l02686 P~ o5 ,lss 2~ 7~3 1 polytetrafluorethylene) to prevent flow-through of catholyte 2 Cathode lead 37 is electrically connected to the support 3 material 31 4 Ar external circuit means ~not shown~ provides an electrical pathway between anode 11 and cathode 30 6 Ir. operation, a fresh aqueous alkaline medium is 7 introduced through inlet 21 into anode chamber 17 to refresh 8 the anolyte, which is constantly being depleted of hydroxyl g ion in accordance with half-cell reaction ~1) 20H - ~ ~C2 + H20 + 2e- (1) 11 and which does not receive an adequate flow of hydroxyl ion 12 from the cathode compartment 19, even under the most ideal 13 circumstances, since the half-cell reaction occurring at the 14 cathode, for every two electrons accepted, produces only one mole of hydroxyl ion in accordance with half-cell reaction 16 ~2) 17 ~2 + H20 + 2e- HO2- + ~~ (2) 18 The electrolyte introduced into anode compartment 17 19 becomes part of the anolyte, but it also passes through the pores of separator 15 into cathode compartment 19 and out of 21 the cell 10 through outlet 25 at a rate selected to limit the 22 limit migration of perhydroxyl ion irto the anode co~r~n~nt 23 17 That is, the direction of flow of electrolyte is 24 r-;n~;n~d ~with the aid of a pump) in an anolyte-to-catholyte direction, which is counter to the natural diffusion of 26 hydroxyl ions from cathode compartment 19 to anode compartment 27 17 and, more importantly, is counter to the tendency of 28 perhydroxyl ions ~HO2-) to mi~rate into the anode chamber 17, 29 where they are exposed to possible oxidation to oxygen This undesirable side reaction, represented below by half-cell 31 reaction ~3) 32 HO2- + OH- - ~ ~2 + H20 + 2e' ~3) SIJBSTITUTE SHEET (RULE 26) ~096/02686 f 1'J~ 'S ~ r~ D~

1 results in the loss of valuable peroxide product and is highly 2 detrimental to the objectives of this invention.
3 The spent catholyte, so to speak, which exits the cell 4 through outlet 2S, i9 the desired product of the electrosynthesis. Thus, outlet 25 serves as the means for 6 recovering the alkaline solution of peroxide which can be used 7 as a bleaching agent or oxidizing agent or solubilizing agent 8 for treating pulp, paper, and other industrial products. The 9 alkalinity and the peroxide content in outlet 25 can be controlled, according to this invention, over a surprisingly 11 broad range by controlling the parameters of cell operation, 12 including electrolyte flow and the like. Generally speaking, 13 the solution in outlet 25 has an alkalinity and a peroxide 14 content which has been carefully matched to the industrial needs prevailing at the site of cell 10, so that cell 10 can 16 serve as an on-site peroxide generator, making just enough 17 peroxide to satisfy current demand, no more and no less. On-18 site generation of peroxide avoids storage of peroxide and 19 purchase of highly concentrated peroxide from outside sources, both of which are undesirable and can even be hazardous.
Z1 The catholyte in cathode chamber 19 flows along the 22 surface of active material 33 of cathode 30 and p~ ~t~q into 23 actlve material 33 to a considerable degree, but does not flow 24 through cathode 30 in the manner catholytes fed to packed-bed or porous matrix cathodes, for several reasons. First, 26 support material 31, though permeable to gas, is hydrophobic 27 and will not permit a~ueous media to pass through it. Second, 28 the volume of active material 31 is very small compared to the 29 volume of a packed-bed cathode, and it will not ac,~ '-te a voluminous flow of liquid. Moreover, the porosity of cathode 31 30 is generally in the form of very fine pores which are 32 better suited to capillary action than high flow rates. In SUBSTIlUTE SHEET (~ULE 26) ~ W096/u2686 ,)~ ~ 4 753 r~ s/~

1 any event, cathode chamber l9 and cathode 30 are designed to 2 provide a fairly rapid flow of catholyte parallel to the 3 surface of cathode 30.
4 The oxygen-cnnt~in;ng gas introduced through inlet 27 contacts the gas-permeable support material 31 of cathode 30 6 and permeates into the active material 33. Because of the 7 permeation of catholyte into active material 33, this portion 8 of cathode 30 provides a multitude of sites for a three-phase g interface of catholyte, oxygen-containing gas, and solid catalytic material The oxygen is reduced to peroxide at this 11 three-phase interface, and the perhydroxyl ions diffuse into 12 the catholyte. Turning now to FIGURE 2, the cell 50 shown in 13 this Figure is preferred for more controllable contact times 14 between catholyte and cathode 30, hence more controllable prod~1rtirn of peroxide. In addition, the cell design shown in 16 FI~URE 2 may allow for the decoupling of current efficiency 17 and product ratio due to the ;n~Pr~n~Pnt catholyte and anolyte 18 flows. The flow of catholyte through cathode compartment 19 19 is rapid enough to prevent any significant migration of perhydroxyl ion into anode chamber 17, thereby eliminating the 21 need for a countercurrent flow of electrolyte. In this 22 embodiment, fresh anolyte (from source 14) of ;n~PrPn~ntly 23 selected alkalinity continuously enters through inlet 22, 24 thereby keeping the hydroxide ion crnrPn~ration in anode compartment 17 from belng excessively depleted, and the spent 26 anolyte flows out through outlet 24. On the cathode side of 27 cell 50, fresh catholyte ~from source 16), also of 28 in~PrPn~Pntly selected ~ 1;n;ty, enters through inlet 26, 29 and the product of the electrosynthesis flows out through outlet 28. The fresh influx cf catholyte through inlet 26 and 31 the constant efflux of catholyte prevent excessive buildup of 32 hydroxide ion in cathode compartment 19, which is very SUBSrlTUTE SH~ET (RULE ~6) WO!J'61~2686 ~1 q d~ 753 r~ s 1 important with respect to T-~inT~jn;n~ proper control over the 2 hydroxyl~perhydroxyl rati.o of the product effluent. As 3 indicated previously, this ratio can be varied from as low as 4 1.6:1 to all higher values. Hydroxyl/perhydroxyl ratios as high as 2:1 or even 1.8:1 are unsuitable for many important 6 industrial uses of alkaline peroxide solution, whereas 7 hydroxyl/perhydroxyl ratio9 as low as 1:1 can create problems 8 in the operation of cells 10 or 50. Accordingly, the 9 particularly preferred hydroxyl/perhydroxyl ratio is in the range of 1.2:1 to 1.7:1.
11 In this invention, it is preferred to utilize the 12 essentially pure oxygen produced at anode 11. This objective 13 is most easily accomplished by circulating the oxygen through 14 a conduit system 41, external to cell 50, to enrich the oxygen-c~nt~;nlng gas introduced through inlet 27 on the 16 cathode side of cell 50. System 41 can be used in cell 10 17 also, but for simplicity of illustration, system 41 is shown 18 only in association with cell 50.
19 Except for the flowing electrolyte arrangement (compare inlet 21 and outlet 25 of cell 10 with inlets 22 and 26 and 21 outlets 26 and 28 of cell 50) it will be noted that cells 10 22 and 50 can otherwise be substantially identical in structure 23 and operation.
24 The loading of gold or gold alloy in the active material 33 of cathode 30 ~of Eigures 1 or 2) can range from 0.1 to 50%
26 by weight, based on the weight of active material 33.
27 ~oadings in the range of 2 to 20~ are preferred. ~etails of 28 the structure of cathode 30 can best be seen in FIGURE 3, 29 which shows hydrophobic support material 31 (optionally treated with a hydrophobic polymer such as 31 polytetrafluoroethylene or a similar fluorinated hydrocarbon), 32 active material 33, and current collector 35. The details SUBSlITUTE SHEET (RULE 26) ~ W096l02686 P~ 51~3 21 q ~~753 1 shown in FIGURE 3 relate to cathode 30 of both Figures l and ; 2 2, because the structure of cathode 30 is identical in both of 3 those Figures.
4 Preferred materials for the separator 15 of FIGURE 1 include alkali-resistant porous inorgAnic oxides and silicates 6 and the like and porous organic polymeric materials which 7 resist strong alkalis, e.g. microporous polyolefins. The 8 separator 23 of FI~URE 2 is a like material, but cation 9 exchange m~ L3lles can also be used in the embodiment of FIGURE 2. The preferred anode 11 is an alkali-resistant bulk 11 metal such as nickel or a noble metal, which is preferably 12 porou~ ~e.g. a metal 9creen or mesh or noYpAn~od metal"). The 13 preferred electrolyte is an aqueous alkaline medium such as an 14 aqueous solution of an alkali metal hydroxide, a highly water-soluble alkaline earth metal hydroxide, or a highly water-16 soluble quaternary ammonium hydroxide. Alkali metal hydroxide 17 solutions are preferred, and sodium particularly preferred 18 from the cost stAn~point.
ls The preferred oXygen-c~ntAininc3 gas introduced through inlet 27 is substantially pure oxygen or a mixture of oxygen 21 with an essentially inert !~as such as nitrogen or argon. A
22 particularly convenient way to obtain a suitable 02/N2 mixture 23 is to remove the carbon dioxide content of air, e.g. through 24 a compression/r~n~PnCAti~n or alkaline-scrub te~hn;~o. Since carbon dioxide can form ~Ar~onAt~ in an alkaline electrolyte, 26 and since some carbonates ~even alkali metal carbonates) can 27 be less soluble than the ~L~L~.".~ing alkali metal hydroxides 28 at cell operating temperatures, resulting in precipitation of 29 ~ArhonAte salt in the pores of the cathode, the presence of carbon dioxide in the electrolyte i~ preferably avoided.
31 The Active M~teri~l 32 It is well known in the art that a gold-con~Aining SUBSTITUTE SHEET (~ULE 26) VfO 96/02686 ~ ? ~3 q ;~ PCT/ITS9!;~09733 1 electrocatalyst of a gas-diffusion electrode in an alkaline 2 electrolyte can facilitate the 9elective reduction of oxygen 3 to peroxide (~2-~ H02-, and/or H202, are all referred to in this 4 application as ~peroxide"), e.g. in accordance with half-cell reaction (2), above.
6 This reaction involves a "two electron change" - as 7 opposed to the "four electron change'~ of the complete 8 reduction of oxygen to hydroxide or water. It is also known 9 that gold crystals can catalyze the four-electron change at the ~100) face of these crystals, whereas the other 11 crystalline faces (and polycrystalline gold) are specific for 12 the two-electron change. See U.S. Patent 5,041,195 ~Taylor et 13 al), issued August 20, 1991, the disclosure of which is 14 incorporated herein by reference. Although gold-containing electrocatalysts could theoretically be very effective in 16 improving the performance of a peroxide electrosynthesis cell, 17 ar.d although gold i9 very stable (resistant to corrosion) in 18 alkaline electrolytes, there appears to be very little 19 discussion in the patent literature regarding the use of such electrocatalysts for this purpose, particulate carbon being 21 the material most typically mentioned as suitable for 22 catalysis of the electrosynthesis. Despite its promise of 23 improved peroxide production, formulation of gold-cnnt~;ning 24 electrocatalytically active materials of suitable efficiency can be problematic, and prior art appears to provide very 26 little detailed guidance in this regard.
27 Surprisingly, the basic principles involved in supporting 28 tiny gold particles on high surface-area carbon, disclosed in 29 the 5,041,195 patent cited above, have ~een found to be highly useful in the context of this invention, even though these 31 principles relate to gold catalysts specific for the four-32 electron reaction (complete reduction to hydroxide or water) SllBS11TUTE SHEET (RULE 26) ~ W096/02686 p,~,~ Jr~ ~3 2 1 ti~ 3 1 rather than the two-electron reaction (partial reduction to 2 peroxide). It has been found that relatively minor 3 modifications of the techni.ques described in the 5,041,195 4 patent can provide gold or gold alloy particles which selectively catalyze the two-electron, peroxide-forming 6 reaction.
7 The techniques of the 5,041,195 patent are directed 8 toward maximizing the formation of tiny monocrystals g (averaging less than 50 A, more typically ~ 40 A, in size1 which are selective for the four-elect.ron reaction. To obtain 11 supported gold or gold alloy particles which are specific for 12 the two-electron change but are otherwise prepared in 13 accordance with the 5,041,195 patent, one can utilize 14 graphitized carbon as the support material and/or select conditions favoring the formation of somewhat larger metallic 16 particles (i.e. particles having an average size of at least 17 40 A, but generally less than 200 A and preferably about 50 to 18 about 150 A?. It has been found that the tiny monocrystalline 19 particles produced accordin~ to the 5,041,195 patent can serve as nucleation sites for the "growthR of somewhat larger 21 particles of almost any desired size within the aforementioned 22 preferred range of 50 to 150 A. For example, the technique 23 described in the 5,041,195 patent can be followed exactly, and 24 the resulting gold-crn~Ainin~ nucleation sites can be subjected to a heating step which sinters together some of 26 tiny metallic particles, thereby increasing their average 27 particle size to ~ 50 A t~ 5 nm?.
28 Suitable catalyst support materials include high surface 29 area carbon and other finel~ divided inorg_nic materials (e.g.
metallic oxides or the li?ce?. Finely divided carbon is 31 presently preferred due to its co~mercial avAilAhility in a 32 range of particle sizes and due to its desirable inherent SUBSrlT~TE SHEET IRllLE 26~

W0 96/~2.f,86 ~ r~ s~ --~ ? '7'1'~ 7 ~ 3 1 catalytic properties. When measured by the B.E.T. method, 2 carbon powders such as furnace blacks, lamp blacks, acetylene 3 blacks, channel blacks, and thermal blacks can pro~ide surface 4 areas ranging from 50 m2/g up to almost 2000 m2/g, surface areas >200 m2/g, e.g. ~600 m2/g being preferred. The particle 6 sizes of the carbon in these powders can range from about 5 to 7 about 1000 nanometers ~5Q to lO,oCo A) but are preferably & smaller than 300 nanometers in size. Since the surface area g of the gold or gold alloy particles is normally less than that of the high surface area carbon, at least some carbon is 11 exposed to the alkaline electrolyte and is subject to chemical 12 attack, but adequate stability in alkaline media can be 13 obtained with cathodes prepared according to this invention.
14 Commercially available carbon materials include BLACK
PEARLS ttrade designation), RETJEN~3LACK ~trade designation), 16 VULCAN (trade designation~, "CSX", and Lurgi blacks, BLACK
17 PEARLS and KETJENBLACR being preferred. These materials are 18 described in detail in U.S. Patent 5,041,195, and this 19 description is specifically included in the subject matter incorporated by reference from the 5,041,195 patent.
21 As indicated above, the preferred method for depositing 22 ~forming in situ) gold or gold alloy particles on high surface 23 area carbon i5 a firatinn of the methods ~;~r]ns~d in U.S.
24 Patent 5,041,195. Generally speaking, a reducible gold ~ in solution i9 impregnated into the support material 26 with the aLd of a polar solvent ~e.g. an alcohol or 27 alcohol/water mixture) having adequate wetting characteristics 28 with respect to the support material; the solvent is gently 29 evaporated at moderate temperatures ~higher temperatures can be used to obtain somewhat larger gold particles); and the 31 resulting dry or substantially dry material is subjected to 32 chemical reduction with a reducing gas such as substantially SUBSTITUTE SHEET ~RULE 26) ~ wog6l026a6 ~ ~ f ~ [;3 r~ 3 1 dry hydrogen. The reducible gold compound can be chlorauric 2 acid ~HAuCl~), a 'salt of this acid, a gold halide, or the 3 like. The resulting gold particles can be smaller than 40 A
4 in size, but, as indicated above, they can also serve as nucleation sites for further particle growth (e.g. by 6 sintering, as indicated previously~. The most preferred 7 active material thus comprises a rather high-surface area gold 8 deposited on an even higher surface area carbon.
g As indicated previou91y, the active material preferably contains at least 30~, generally about 50 to 70 weight-% of 11 polymeric hydrophobic binder, The binder can be introduced 12 into the active material, during its preparation, as a 13 suspension of fine particles of hydrophobic polymer in a 14 carrier such as water or an organic solvent.
The peroxide yield is variable and can readily provide 16 the range of 3-5 wt% typically desired by the pulp and paper 17 industry.
18 The following Example illustrates the principle and 19 practice of this invention without in any way limiting its scope.
21 PRrt A - Cathode Pr~rRration Step 22 Cathodes were prepared as gas-diffusion electrodes 23 (GDE'~) from carbon-fiber paper as the gas-permeable support 24 layer and a single deposit of an electrocatalyst layer (elecL~ Rl ly active material). The preparation process 26 was begun by first sieving the particulate carbon, which is 27 itself an but was used as catalyst support material in this 28 Example, through a -170 mesh (U.S. or Tyler) screen. The 29 particulate carbon support material was then dispersed in an acidified aqueous solution (65 millimolar sulfuric acid).
31 Ultrapure water was used to make this acidified aqueous 32 solution. The carbon was added to the acidified aqueous SUBSIITUTE SHEET (RULE 26) W096/02686 ~ I q~ r~

1 solution with stirring and ultrasonification such that when 2 the entire solution was applied to the carbon-fiber paper the 3 electrocatalyst loading was 5 mg/cmZ. For each 10 cm x 10 cm 4 of electrode area the carbon mass was typically O.S17 grams.
A polytetrafluoroethylene lPTFE) binder was then added (30, 6 S0, or 70 wt~), using a dilute aqueous suspension of "TFE-30"
7 (trade designation of the DuPont Company), and the suspension 8 was mixed with stirring and ultrasonification. The resulting 9 blend was filtered and the filtrate ldepcsit on the filter paper) was transferred onto wet-proofed, porous carbon-fiber 11 paper substrate (Toray Industries) to form a uniform layer.
12 The electrode was subse~uently cold pressed at 1200 pounds 13 pressure for each 16 in7 of electrode area (7S psi, 517 kPa) 14 until dry, hot pressed up to 1200 pounds pressure (75 psi, S17 lS kPa) for five minutes at 100 C, and sintered stepwise at 100 16 C for one hour, 200 C for an hour, then finally at 300 C for 17 fifteen minutes.
18 To produce an electrocatalyst layer with small gold 19 particles required the previously-described modification to the process described in patent S,041,19S. This patent 21 details the techniques directed toward maximizing the 22 f~nm~ti~n of tiny monocrystals (averaginy less than S0 A, more 23 typically c 40 A, in size) which are selective for the four-24 electron reaction. To obtain supported gold or gold alloy partioles which are specific for the two-electron change but 26 are otherwise prepared in accordance with the S,041,19S
27 patent, one can utilize graphitized carbon as the support 28 material and/or select conditions favoring the formation of 29 somewhat larger metallic particles li.e. particles having an average size of at least 40 A, but generally less than 200 A
31 and preferably about S0 to about lS0 A). It has been found 32 that the tiny . - y~Lalline particles produced according to SUBSTITUTE SHEET ~RUI.E 26) ~ ~h'0 96~02686 ,~ , r P~ ~ ., /J.~
) 3 lg 1 the 5,041,195 patent can serve as nucleation sites for the 2 "growth" of somewhat larger particles of almost any desired 3 size within the aforementioned preferred range of 50 to lS0 A.
4 In this Fxample, the technique described in the 5,041,195 patent can be followed exactly and is incorporated herein by 6 reference, except that the resulting gold-cont~ining 7 nucleation sites were subjected to a heating step in an inert 8 atmosphere at temperatures ranging from 300 to 1200 C which 9 sintered together some of tiny metallic particles, thereby increasing their average particle size to ~ 50 A (, 5 11 nm).
12 Part i3 - Per~x;de G~nPrator P~Arat~c 13 Two size systems were used to test and evaluate the 14 cathodes and their performance. Both types employed three-~UIII~L i ~, flowing electrolyte designs as shown in Figure 2 16 of the Drawing. The smaller size was original with the 17 present applicants and was used for small cathodes of area not 18 exceeding 3.0 cm2. These cells were constructed from 19 polymethacrylate (LUCITE2\ with a total cell volume of approximately 10 ml. A constant electrolyte flow was 21 malntained by a peristaltic pump and the electrolyte was 22 recirculated from a continuously mixed 600 ml reservoir. As 23 shown in Figure 2, the electrolyte ,- I ~ was partitioned 24 into two separate compartments by a cation exchange membrane, Nafion~ 117 ~trademark of the DuPont Company). The anode 26 electrode was a solid nickel sheet. The electrolyte 27 temperature was ~~;nt~;ned above 40 C by feedback-controlled, 2e in-line heaters through which the electrolyte passes.
29 Blectrolyte temp~rature was monitored by insertion of thermocouples into the fluid entrances and exits. A
31 mercur~/mercury oxide electrode was used as reference. The 32 internal resistance of the cells was monitored through the use SUBSrITUTE SHEET (~ULE 26~

U096/02686 ~} J ~ 3 ~ r~ J~

1 of an auxiliary platinum wire electrode in combination with an 2 800 IR Measurement System (Electrosynthesis Corp.). The cells 3 were operated in constant current mode with a 3-Amp power 4 supply. Four such cells were constructed and typically operated simultaneously with a computer-based data ac~uisition 6 system.
7 A larger cell system with cathode area up to 100 cm~ was 8 prepared by adaptation of a commercial electrolyzer This 9 system was referred to as a process development unit (PDU).
and comprised an EA ElectroCell MP (EA Corp., Sweden, ~ht~ir~d 11 through the Electrosynthesis Corp.), suitably modified as 12 described below. This cell was chosen over other possible 13 commercial alternatives as it is more readily adapted for gas 14 diffusion electrodes. The principal difference between this and the small apparatus was the variable cathode-anode 16 spacing. The power supply was proportionately larger and was 17 purchased from Power Ten Inc. In addition to a cation 18 exchange membrane, Nafion~ 117 (DuPont), an alternative, 19 relatively inexpensive separator was also successfully used in the PDU. This was 7-mil (175-~m) thick Teslin~ ~PPG
21 Industries Inc.) which is a silica-based, porous, polymeric 22 material. The Teslin~ separator was determined to perform as 23 well as the Nafion~ 117 membrane. Several changes to the EA
24 ElectroCell MP (commercia]. system) were nocPss~ry to fully adapt it to serve as the PDU (unit for caustic peroxide 26 generation via GDE's). These changes were:
27 (1~ The picture frame cathode assembly was replaced with a 28 machined graphite block. The addition of the graphite 29 block provides physical support of the carbon backing layer necessary to compensate for hydrostatic pressure.
31 The block also ensures a leak-free seal between the 32 catholyte and gas compartments. We chose graphite to SUeSTlTUTE S~EET ~RULE 26~

~ W096/02686 r~ "~
2 t ~ 3 1 provide electrical contact and for corrosion resistance.
2 The block was drilled with a matrix of holes to allow gas 3 to uniformly contact the rear surface of the GDE. This 4 matrix of holes accounted for approximately lO~ of the active cathode area. Therefore, the cathode-side of the 6 block was ~Ch;n~ to form ch~nnels leaving only small 7 pegs in which to contact the GDE. Total surface area of 8 these pegs was approximately 10~. The channeling 9 provided uniform gas-flow across the back side of the GDE
and 9o~ cathode area utilization.
11 12) The separator frames were replaced with c on~nrs more 12 compatible with elevated temperature operation. These 13 frames define the cathode-anode gap and have mesh inserts 14 which promote turbulence. The replacement frames were ecluipped with reinforced flow canals to resist 16 deformation under pressure and elevated temperature.
17 13~ The solid nickel sheet anode was drilled with a matrix of 18 holes to resemble a mesh. This modification was 19 n~c~=s~y to allow evolved gas to escape behind the anode, away from the membrane.
21 (4~ Accurate temperature monitoring was also rec~uired. This 22 was accomplished by insertion of Teflon~-tipped 23 thermocouple probes directly into the caustic entrance 24 and exit ports of the P W .

(5) Shut-down of the PDU was necessary for changing gas 26 cylinders and electrolyte and for short-term unattended 27 operation, such as cathode break-in. At the open circuit 28 potential, the cathode begins to oxidize immediately.
29 Removing the electrolyte was insufficient to prevent this, since the cathode and memorane retain moisture for 31 long periods. The PDU stand was modified to allow 32 draining and flushing of the cell with deionized water.

SVBSTITUTE SHEET ~RULE 2B) ~096~2686 ~ ' 5 ~ r~ v s~ J

1 A four-way valve was also added to the gas feed s~stem to 2 allow the cell to be nitrogen-purged to displace oxygen.
3 P~rt C - Perfon~-n~e Data 4 Cathodes c -sed of up to 10 wt% gold on ~etjenBlack (trade designation for particulate carbon) were prepared as 6 described in Part A and were mounted in the PDU apparatus as 7 described in Part B. The system was operated at cell 8 temperatures of 45-50 C, with a total anode to cathode gap of 9 0.8 cm, and for an input electrolyte concentration of 10 wt%
NaOH. Cell polarizations ~i.e. total cell potential versus 11 current density) of the yold on carbon electrodes were equal 12 to or improved in comparison to the carbon black alone. In 13 addition, the gold-catalyzed electrodes were capable of 14 sustained operation at current densities equal to or ~Y~ee~;ns 300 mA/cm2. The performance of a 10 wt% gold-on-carbon 16 electrode was 1.34 V at 100 mA/cm2, 1.73 V at 200 mA/cm2, and 17 2.08 V at 300 mA~cmZ.
18 The reporting operating conditions of the trickle bed 19 cathode system are 2 V at 60 mA/cm2. The operating potential for the gold-on-carbon electrode tested here is approximately 21 1.06 V at 60 mA/cm2. Thus, for the same current density a 22 factor of two in power savings per part by weight of peroxide 23 product can be realized. Alternatively, higher current 24 operation may be perfonned to reduce overall system size thereby reducing the generator capital costs. The 300 mA/cm2 26 current density data indicates that in comparison to the 27 trickle bed system, a GDE-based generator system may be 28 reduced in total electrode area by up to a factor of five.
29 This will translate to a significant reduction in generator system capital costs since these costs are a linear func~ion 31 of electrode area. Further perfonmance ; ~VG n~S, 32 principally due to decreases in the IR losses, are expected as SUBSTITUTE SHEET (~ULE 26~

~ W096l0~68C r ~ 1 q '~ 7 5 ~7) l the gap is decreased to a reasonable commercial limit of C.5 2 cm.
3 Cathode lifetime and cathode cost are critical parameters 4 in determining commercial system operating costs. The more frequently the cell5 have to be replaced and the more 6 expensive the cells c08t, the higher the cost of the 7 electrogenerated peroxide will be and the poorer the 8 comparison will be with simple purchase and storage 9 Extensive teating of the electrodes in both size systems described in Part B have identified total electrode 11 hydrophobicity to be a critical factor in determining cathode 12 lifetime. The most successful cathodes have incorporated 13 hydrophobic polyhalohydLo~aLLon polymer, preferably PTFE, into 14 the carbon paper and contain ~30 wt.-~ PTFE le.g. 50 and 70 wt.-%) in the electrocatalyst layer, i.e. in the 16 electrochemically active material. With active material 17 containing S0 to 70 wt.-~ PTFE, the cathodes will survive for 18 several thousand hours with only minor decreases in 19 performance characteristics.
In view of the relatively low cost of the catalyst 21 components described above, the use of gold in the active 22 material does not increase the manu~acturing cost of the gas-23 diffusion cathode beyond present goals for on-site peroxide 24 generator markets in the U.S. and elsewhere.

SU~STITLITE SHEET [RULE ~6~

Claims

What is claimed is:
1. An electrochemical cell for the production of an alkaline solution of peroxide, comprising:
a partitioning means for partitioning said electrochemical cell into an anode compartment and a cathode compartment, an anode compartment containing an anode and an aqueous alkaline anolyte, a cathode compartment defining a space containing a generally free-flowing aqueous alkaline catholyte, a gas-diffusion cathode having two major surfaces, said cathode occupying not more than a minor proportion of the volume of said space, said cathode compartment being constructed and arranged to permit generally unrestricted flow of the aqueous alkaline catholyte across a first major surface of said gas-diffusion cathode, said cathode compartment being provided with means for withdrawing peroxide dissolved in said catholyte from said space defined by said cathode compartment, the second major surface of said cathode being gas-permeable and being in contact with an oxygen-containing gas, said first major surface of said cathode comprising an electrochemically active material, said active material comprising:
a particulate catalyst support material having a surface area, by the B.E.T. method, of about 50 to about 2000 m2/g, deposited on the particles of catalyst support material of said active layer, 0.1 to 50 weight-%, based on the weight of the active layer, of a particulate elemental metal comprising gold particles having an average size, measured by transmission electron microscopy, which is greater than about 4 but less than about 20 nanometers, said particulate elemental metal being substantially selectively catalytic for the reduction of oxygen to peroxide ion or hydrogen peroxide, and electrical leads in electrical contact with said anode and said gas-diffusion cathode.
2. The electrochemical cell according to claim 1, wherein said partitioning means comprises a liquid-permeable separator for permitting diffusion of anions from said catholyte compartment into said anolyte compartment.
3. The electrochemical cell according to claim 1, wherein said anode compartment includes inlet means for introducing aqueous alkaline anolyte thereunto and outlet means for permitting aqueous alkaline anolyte to flow out of said anode compartment, and wherein said cathode compartment includes inlet means for introducing aqueous alkaline catholyte thereunto.
4. The electrochemical cell according to claim 1, wherein said peroxide is in the form of perhydroxyl ion, HO2.
5. The electrochemical cell according to claim 1, wherein the ratio of hydroxyl ion to perhydroxyl ion in the catholyte withdrawn from said cathode compartment is less than 2:1.
6. The electrochimical cell according to claim 1, wherein said electrochemical cell is a bipolar cell which is electrically connected to at least one other electrochemical cell of substantially the same construction and having substantially the same mode of operation.
7. The electrochemical cell according to claim 1, wherein said gas-diffusion cathode comprises: a gas-permeable support layer providing said second major surface, said active material being supported on said gas-permeable support layer, wherein the particulate catalyst support material of said active material has a surface area, by the B.E.T. method, of at least about 200 ml/g; and wherein said gold particles have an average size, measured by transmission electron microscopy, which is greater than 5 nanometers.
8. The electrochemical cell according to claim 1, wherein said anode comprises an alkali-resistant bulk metal.
9. The electrochemical cell according to claim 1, wherein said first major surface of said cathode comprises an electrochemically active material comprising:
said particulate catalyst support material, deposited on the particles of catalyst support material, said particulate elemental comprising gold particles, and blended with said particulate catalyst support material, about 50 to 70% by weight, based on the weight of the electrochemically active material, of a hydrophobic polymer.
10. A process for producing an alkaline solution containing peroxide ion in an electrochemical cell, characterized in that:
a) one operates the electrochemical cell at a voltage which can vary with the current density but does not exceed about 2 volts for a current density of at least 200 mA/cm2, and said electrochemical cell comprises a partitioning means for partitioning said electrochemical cell into an anode compartment and a cathode compartment;
an anode compartment containing an anode and an aqueous alkaline anolyte; a cathode compartment defining a space containing a generally free-flowing aqueous alkaline catholyte, a gas-diffusion cathode having two major surfaces, said cathode occupying not more than a minor proportion of the volume. of said space; the second major surface of said cathode is gas-permeable and is in contact with an oxygen-containing gas, so that the oxygen-containing gas can be contacted with said cathode essentially independently and separately from said generally free-flowing aqueous alkaline electrolyte, said first major surface of said cathode comprises an electrochemically active material, which active material comprises:
a particulate catalyst support material having a surface area, by the B.E.T. method, of about 50 to about 2000 m2/g, on the particles of catalyst support material of said active layer, 0.1 to 50 weight-%, based on the weight of the active layer, of a particulate elemental metal comprising gold particles having an average size, measured by transmission electron microscopy, which is greater than about 4 but less than about 20 nanometers;
said particulate elemental metal is substantially selectively catalytic for the reduction of oxygen to peroxide ion or hydrogen peroxide;
and electrical leads in electrical contact with said anode and said gas-diffusion cathode;
b) one causes said generally free-flowing catholyte to flow across said first major surface of said cathode;
c) one contacts said second major surface of said cathode with an oxygen-containing gas, thereby forming a multi-phase interface within said cathode, said interface comprising said active material, said oxygen-containing gas, and said catholyte, d) one reduces oxygen to peroxide at said multi-phase interface and permits the resulting peroxide to diffuse into said catholyte, and e) one withdraws from said cathode compartment an alkaline solution of peroxide in which the hydroxyl:perhydroxyl ratio does not exceed about 2:1.
11. The process according to claim 10, wherein an aqueous alkaline medium for providing an anolyte is introduced into said anode compartment from a source of alkaline medium external to said electrochemical cell.
12. The process according to claims 10 or 11, wherein an aqueous alkaline medium for providing a catholyte is introduced into said cathode compartment from a source of alkaline medium external to said electrochemical cell.
13. The process according to claim 10, 11, or 12, wherein said electrochemical cell is operated at an overall cell voltage of less than 2 volts, a current density of at least about 200 mA/cm2, and a current efficiency of at least about 85%.
14. The process according to claim 10, 11, 12, or 13, wherein oxygen produced by oxidation of hydroxide ion at the anode is collected and introduced into the oxygen-containing gas contacting the second major surface of said cathode.
15. The process according to claim 10, 11, 12, 13, or 14, wherein the alkaline solution of peroxide withdrawn from the cathode compartment is spent catholyte formed as a result of said reducing step and is the desired product of the process.
15. The process according to claim 10, 11, 12, 13, 14, or 15, wherein said electrochemical cell is operated at a temperature of at least about 35°C.
17. The process according to claim 10, 11, 12, 13, 14, 15, or 16, wherein the amount of said particulate elemental metal deposited on the particles of catalyst support material ranges from 0.1 to 10 weight-%.
18. The process according to any of claims 10 through 17, wherein said process is carried out for a period of time exceeding a thousand hours, and wherein said gas-diffusion cathode resists chemical attack by the aqueous alkaline catholyte for said period of time.
19. The process according to any of claims 10 to 18, wherein said particulate elemental metal is in the form of particles smaller in size than 5 nm which have been combined together to form particles larger than 5 nm.